Hardpan Layer Research Articles

Influence of hardpan layers on arsenic mobility in historical gold mine tailings

Abstract Hardpans, or cemented layers, form by precipitation and cementation of secondary minerals in mine tailings and may act as both physical and chemical barriers. Precipitation of secondary minerals during weathering of tailings can sequester metal(loid)s, thereby limiting their release to the environment. At Montague Gold Mines in Nova Scotia, tailings are partially cemented by the Fe arsenate mineral scorodite (FeAsO4·2H2O). Previous studies have shown that the formation of scorodite can effectively limit aqueous As concentrations due to its relatively low solubility (

Potential risk of calcium carbonate precipitation in agricultural drain envelopes in arid and semi-arid areas

Soil particle deposition and/or chemical precipitation can reduce the permeability of drain envelopes and filters. The first step in recognizing clogging phenomena is the identification of the nature of the precipitating materials. Calcium carbonate is a substance of low solubility that precipitates rapidly, forming a hard pan layer in the soil and/or clogging drain envelopes. The main objective of the present study is to investigate the precipitation risk of calcium carbonate in agricultural drain envelopes in the Khuzestan province of Iran. In this study, three indicators namely Ryznar, Langelier and Stiff-Davis indices were employed to assess the precipitation risk of calcium carbonate in agricultural drainage water. Results showed that all agricultural drainage systems in the study area give evidence of a potential risk of calcium carbonate precipitation, but the severity of the problem is different. The results also showed that the Ryznar and Stiff-Davis indices provide a better estimation of the potential precipitation risk of calcium carbonate than the Langelier index. Analyses of soil samples and drain envelopes from a drainage system , installed in the Abadan palm grove, showed that the main chemical component was calcium carbonate.

Micro paddy lysimeter for monitoring solute transport in paddy environment

The micro paddy lysimeter (MPL) was developed and evaluated for its performance to simulate solute transport in paddy environment under laboratory conditions. MPLs were constructed using soil collected from Field Museum Honmachi of Tokyo University of Agriculture and Technology, Japan. For the physical characteristics of the hardpan layer, parameters such as thickness, and soil aggregate size, affecting the percolation rate were studied. For the plow layer, two types of plow soils, sieved and un-sieved soils were compared. The sieved soil plow layer was produced by mixing air-dried soils of different aggregate sizes of D > 9.50, 9.50 ≥ D > 4.75, 4.75 ≥ D > 2.0 mm and D ≤ 2.0 mm at 47.1, 19.5, 20.6, and 12.8%, respectively. The un-sieved plow layer soil was directly used after collecting from the field. Inert tracer was applied to ponding water with controlled boundary conditions to evaluate the reproducibility of the soil hydraulic characteristics. HYDRUS-1D was used to evaluate the movement of bromide tracer in the MPL. The proposed conditions of the MPL were that the hardpan layer can be made from soil aggregates smaller than 0.425 mm with 2 cm thickness and that the plow layer can be prepared with sieved or un-sieved soils. With these conditions, the obtained results proved that MPLs can be a useful tool to simulate solute transport in paddy environment.

ASPECTS REGARDING THE REDUCTION OF PENETRATION RESISTANCE OF SOIL AS A CONSEQUENCE SOIL WORKING

Soil penetration resistance greatly affects crop quality, the foresight resistance of various working parts of farm machinery and therefore energy consumption for the establishment of different cultures. The paper presents results of experimental determination of soil penetration resistance before and after deep raising work, made with the subsoiler. This paper presents the positive effects upon the soil structure in depth and to surface, the restauration of water circulation and the improvement of aeration improving in this way the development of the radicular system of plants. Also, in this paper it is reduced the hard-pan layer which leads to a decrease of energetic consumption to the other works which needs to be done with the role of attendance and presence of crops. The experimental researches have been developed within The National Institute of Mechanics of Agriculture from Bucharest, on three different types of soil. The raising equipment used for the experimental researches was equiped with working parts type flat chisel, and the soil compaction meter used to determine the penetration resistance to penetration is portable static type, made by Dickey John firm from United States of America. The determination of penetration resistance was made also before the work of deep raising and also after it, in this way being established the influence of this work upon the soil properties.

Nitrogen Leaching in Intensive Cropping Systems in Tam Duong District, Red River Delta of Vietnam

The environmental and economic conse- quences of nitrogen (N) lost in rice-based systems in Vietnam is important but has not been extensively studied. The objective of this study was to quantify the amount of N lost in major cropping systems in the Red River Delta. An experiment was conducted in the Red River Delta of Vietnam, on five different crops including rose, daisy, cabbage, chili, and a rice-rice- maize rotation during 2004 and 2005. Core soil samples were taken periodically in 20-cm increments to a depth of 1 m and analyzed for nitrate-nitrogen and ammonium-nitrogen. The results indicate appre- ciable leaching losses on N in high-rainfall and irrigation conditions, especially when fertilizer appli- cation was not well synchronized with crop N demand. Highest annual leaching losses of N were recorded in flowers with 185-190 mm of percolation and 173-193 kg N ha −1 , followed by vegetable (cabbage and chili) with 120-122 mm of percolation and 112-115 kg N ha −1 , while it was lowest in rice with about 50 kg N ha −1 . We developed a simple N transport model that combined water and N move- ment through the soil profile. In most cases, the model accurately predicted the seasonal dynamics of N as well as N flow between soil layers and the amounts of N lost from the soil profile. The simulated results of N leaching with soil "puddling" conditions illustrate the advantage of an impermeable or hardpan layer in increasing water and nutrient use efficiencies in these soils. These model results also showed that it is possible to accurately estimate N losses with only a few parameters and helped us identify the risks of N leaching.

Modelling of pneumatic tractor tyre interaction with multi-layered soil

A non-linear finite element (FE) model of the interaction of a tractor tyre with soil surface was developed as a procedure for analysing the contact pressure distribution for varying wheel loads and tyre inflation pressures. The tyre model considered the geometry and orientations of the tyre cords in each ply, the near-incompressible property of the tread rubber block, and the high and non-linear deformation of the carcass under the tread. The soil model was simulated with a linear model and considered a realistic soil cross-sectional area of influence with layers separation, including three soil surface layers and a hardpan layer. The three surface layers were within normal tillage depth (soil surface to 250 mm depth), with 250-300 mm hardpan substratum. Tyre-soil interaction model was developed using ANSYS FE software and the results, which describe the influence of the wheel load and tyre inflation pressure on contact pressure distributions were analysed. Results from the simulation were compared with the measured data to verify the validity of the model for soft soil conditions. The comparison showed reasonably good agreement between the computed and measured general pattern of the pressure distribution at the tyre-soil interface under different loads and inflation pressures.

Non-linear finite element analysis of cone penetration in layered sandy loam soil – Considering precompression stress state

Axisymmetric finite element (FE) method was developed to simulate cone penetration process in layered granular soil. The FE was modeled using ABAQUS/Explicit, a commercially available package. Soil was considered as a non-linear elastic plastic material which was modeled using variable elastic parameters of Young’s Modulus and Poisson’s ratio and Drucker–Prager criterion with yield stress dependent material hardening property. The material hardening parameters of the model were estimated from the USDA-ARS National Soil Dynamics Laboratory – Auburn University (NSDL-AU) soil compaction model. The stress–strain relationship in the NSDLAU compaction model was modified to account for the different soil moisture conditions and the influence of precompression stress states of the soil layers. A surface contact pair (‘slave-master’) algorithm in ABAQUS/Explicit was used to simulate the insertion of a rigid cone (RAX2 ABAQUS element) into deformable and layered soil medium (CAX4R ABAQUS element). The FE formulation was verified using cone penetration data collected on a soil chamber of Norfolk sandy loam soil which was prepared in two compaction treatments that varied in bulk density in the hardpan layer of (1) 1.64 Mg m −3 and (2) 1.71 Mg m −3. The FE model successfully simulated the trend of cone penetration in layered soils indicating the location of the sub-soil compacted (hardpan) layer and peak cone penetration resistance. Modification of the NSDL-AU model to account for the actual soil moisture content and inclusion of the influence of precompression stress into the strain behavior of the NSDL-AU model improved the performance of FE in predicting the peak cone penetration resistance. Modification of the NSDL-AU model resulted in an improvement of about 42% in the finite element-predicted soil cone penetration forces compared with the FE results that used the NSDL-AU ‘virgin’ model.

Simulation of retention and transport of copper, lead and zinc in a paddy soil of the Red River Delta, Vietnam

Heavy metal (HM) contaminations in the topsoil around handicraft villages with non-ferrous heavy metal recycling in the Red River Delta can impose serious threats to the subsoil as well as to the groundwater quality. This feature is very important for paddy soils due to relatively high leaching rates and the dissolution of Fe–Mn oxides under reducing conditions which can accelerate the amount of HM translocated to the subsoil and groundwater. The transport of Cu, Pb and Zn in paddy soils was simulated by numerical modeling of non-equilibrium solute transport with an adaptation of the Hydrus-1D model. For the simulation, a water layer on the soil surface was included, from which HM can infiltrate into the soil depending on the soil hydraulic properties. Sorption coefficients, obtained from batch experiments were used as input data for the simulations. Calculated leaching rates were compared with the binding forms of HM in the samples. The simulations show that leaching rates decrease in the order: Zn > Cu > Pb. This order is confirmed by the results of sequential extractions. Under constant flooded conditions at a water table of 20 cm, Cu, Pb and Zn were estimated to reach the soil depth of 1 m within 470, 495 and 370 days, respectively, emphasizing that reactive pollutants can reach groundwater in a relatively short time. A change of the water layer from 1 to 30 cm can accelerate the leaching rate of HM up to 36%. The hard pan layer was observed to induce a hysteresis in hydraulic conductivity and slow down the movement of HM. Uncertainties in modeling arise as several parameters in the simulation can be determined only with significant errors. However, Hydrus-1D is a suitable tool for simulation of the transport of HM in paddy soils.

Benefits of site-specific subsoiling for cotton production in Coastal Plain soils

The negative impacts of soil compaction on crop yields can often be alleviated by subsoiling. However, this subsoiling operation is often conducted at unnecessarily deep depths wasting energy and excessively disturbing surface residue necessary for erosion control and improved soil quality. A corn ( Zea mays L.)–cotton ( Gossypium hirsutum L.) rotation experiment was conducted over 4 years on a Coastal Plain soil with a hardpan in east-central Alabama to evaluate the potential for site-specific subsoiling (tilling just deep enough to eliminate the hardpan layer) to improve crop yields while conserving energy. Seed cotton yield showed benefits of subsoiling (2342 kg/ha) compared to the no-subsoiling treatment (2059 kg/ha). Averaging over all years of the study, site-specific subsoiling produced cotton yields (2274 kg/ha) statistically equivalent to uniform deep subsoiling at a 45 cm depth (2410 kg/ha) while not excessively disturbing surface soil and residues. Significant reductions in draft force were found for site-specific subsoiling (59% and 35%) as compared to uniform deep subsoiling at a 45 cm depth in shallow depth hardpan plots (25 cm) and medium depth hardpan plots (35 cm), respectively. Calculated fuel use for site-specific subsoiling was found to be reduced by 43% and 27% in the shallow and medium depth hardpan plots, respectively, as compared to uniform deep subsoiling in these same plots. Producers in the Coastal Plains who can determine (or who know) the depth of their root-impeding layer and perform site-specific subsoiling can have comparable cotton yields to traditional uniform depth subsoiling with reduced energy requirements.

PERKEMBANGAN KONSEP PENURUNAN TAHANAN TARIK PADA PENGEMBANGAN DISAIN BAJAK SUBSOIL GETAR

From many experiments with different kinds of tiIIage tools it is well known that an oscilating or vibrating movement of a tool can have a considerable influence on the draft required. Subsoiler represent tools of deep tillage, the function of subsoil plough to cutting deep tillage and to breaking hardpan soil. The hardpan soil can hinder drainage, aeration and pursue spreading of the soil nutrition. Therefore in split/cut the hardpan layer require by big enough of energy. This operation requires heavy-duty tractor because of high draft consumption, especially for breaking subsoil up to depth of 50 cm. Vibratory technique is one of the methods, which could be applied in order to reduce draft. On the other hand reduced draft can used by vibration, mechanism of vibration exploitedly with rotation of PTO changed become translation movement. However that vibration can greatly reduce the draft force needed, to pull a blade or tine trough soil, but this can only be achieved at the expense of an increase in the total powor consumed. Diterima: 12 Pebruari 2007, Disetujui:. 3 Mei 2007

Is it possible to manipulate root anchorage in young trees?

The optimal root system architecture for increased tree anchorage has not yet been determined and in particular, the role of the tap root remains elusive. In Maritime pine (Pinus pinaster Ait.), tap roots may play an important role in anchoring young trees, but in adult trees, their growth is often impeded by the presence of a hard pan layer in the soil and the tap root becomes a minor component of tree anchorage. To understand better the role of the tap root in young trees, we grew cuttings (no tap root present) and seedlings where the tap root had (−) or had not (+) been pruned, in the field for 7 years. The force (F) necessary to deflect the stem sideways was then measured and divided by stem cross-sectional area (CSA), giving a parameter analogous to stress during bending. Root systems were extracted and root architecture and wood mechanical properties (density and longitudinal modulus of elasticity, EL) determined. In seedlings (−) tap roots, new roots had regenerated where the tap root had been pruned, whereas in cuttings, one or two lateral roots had grown downwards and acted as tap roots. Cuttings had significantly less lateral roots than the other treatments, but those near the soil surface were 14% and 23% thicker than plants (+) and (−) tap roots, respectively. Cuttings were smaller than seedlings, but were not relatively less resistant to stem deflection, probably because the thicker lateral roots compensated for their lower number. Apart from stem volume which was greater in trees (+) tap roots, no significant differences with regard to size or any root system variable were found in plants (−) or (+) tap roots. In all treatments, lateral roots were structurally reinforced through extra growth along the direction of the prevailing wind, which also improved tap root anchorage. Predictors of log F/CSA differed depending on treatment: in trees (−) tap roots, a combination of the predictors stem taper and %volume allocated to deep roots was highly regressed with log F/CSA (R2 = 0.83), unlike plants (+) tap roots where the combined predictors of lateral root number and root depth were best regressed with log F/CSA (R2 = 0.80). In cuttings, no clear relationships between log F/CSA and any parameter could be found. Wood density and EL did not differ between roots, but did diminish with increasing distance from the stem in lateral roots. EL was significantly lower in lateral roots from cuttings. Results showed that nursery techniques influence plant development but that the architectural pattern of Maritime pine root systems is stable, developing a sinker root system even when grown from cuttings. Anchorage is affected but the consequences for the long-term are still not known. Numerical modelling may be the only viable method to investigate the function that each root plays in adult tree anchorage.

Combined Soil Physical Stress of Soil Drying, Anaerobiosis and Mechanical Impedance to Seedling Root Growth of Four Crop Species

Soil compaction often creates combined physical stresses of drought, anaerobiosis, and mechanical impedance in field soil. This paper aims to analyze the effect of combined and independent soil physical stresses on crop root growth to find out the species-specific response to the physical stresses, which has not been reported before. Drying stress without the increase of mechanical impedance was evaluated in a very loose pot soil environment. This drying stress did not modify the root elongation rates of rice and pea by the 48 h exposure to the stress environment. For maize and cotton, however, mild drying stress (−80 kPa Ψw) enhanced root elongation by 17-18%, but severe drying stress (−900 kPa Ψw) reduced it by 17-21% as compared with the control environment (−10 kPa Ψw). The combined stress of drying and mechanical impedance nearly stopped the root elongation in all the species, while that of anaerobiosis and mechanical impedance did not stop the elongation of rice and cotton; cotton elongated about 32% of control environment. In maize, root diameter was reduced by the severe drying stress due to the reduction in the number of cortical cell layer and diameters of both central cylinder and xylem vessel. In contrast, cotton showed a significant increment of cortex diameter, although overall diameter was not statistically increased by the severe drying stress. The ability of cotton to continue elongation under anaerobiosis and mechanical stress implied the higher penetration ability to the hard pan layer under the anaerobic condition just after the heavy rainfall.

English

To assess the mobility of metals and consider the geochemical consequences of remediation for exposed Ni-Cu tailings at INCO Ltd., Thompson, Manitoba, Canada, the tailings were described visually, characterized mineralogically, and subjected to sequential extraction and hydraulic conductivity tests. Exposure of the tailings has resulted in the release of metals from sulfides, the acidification of surface and shallow ground waters within the tailings body, and the formation of cemented hardpan layers. Sequential extraction from the solid tailings showed most of the Cu and Zn to be adsorbed by secondary Fe- minerals, while ~15% of the total Ni concentration occurs as water-soluble sulfates in the vadose zone. A rise in water level would result in the dissolution of soluble minerals and the remobilization of metals from the exposed tailings. The formation and thickness of the cemented hardpan layers on the surface depend on the elevation of the water table and the degree of capillary action as this controls the moisture content of the pore space and hence sulfide oxidation. According to geotechnical measurements, the effect of cementation on reducing water flow through the hardpans to the interior of the tailings is insignificant, but the hardpan could act as a diffusive barrier for oxygen. In the hardpans, from 30 to 50% of sulfides are still not oxidized, and are, therefore, able to generate acid. The high acidity of the water at the surface of the exposed tailings prevents metals from being effectively co-precipitated by the secondary Fe-minerals, jarosite and schwertmannite. Thus, capping exposed tailings with acid neutralizing material is recommended.

Penggunaan Persamaan Hiperboloid untuk Identifikasi Struktur Geometri Bajak Singkal

Various geometric structures of moldboard ploughs need to be identified since there have been confusing uses of European and Asian Ploughs in cultivating lowland fields: the European one that was constructed for tillage of upland soil prior to destruct impervious soil layers in root zones has been frequently applied in wetland paddyufields. It leads to frequently disturbances of the formation of hardpan layer structures required for an ideal wetland paddy field that should be formed within a soil depth of 20-40 cm. The research observed three types of moldboard ploughs dominantly operated in wetland paddy filds: a wood (Lanyam), a wood-iron (Brujul) and an iron/steel ploughs. Research method employed a descriptive explanatory one to identify the geometric stucture of the ploughs based on hyperboloid equation: ´2/a2 + Y2/b2-Z2/c2= 1. The plough identity was determined by ratio values amongst the constans a, b and c of the equation, i,e.; R1 = a/b R2 = a/c, and R3 = b/c, where the respective constants were caculated according to the positions of a numbers of coordinate points on the moldboards projected in the Cartesian coordinate system. Results of identification showed thaht the values of R1, R2 and R3 could indicate the identity of the ploughs where Lanyam and Brujul were categorized into the types of Asian ploughs where Lanyam and Brujul were categorized into the types of Asian ploughs by their R1 and R2 3 1, so their operation on wetland paddy fields were reasonable. Whereas the iron/steel one was categorized into the type of European ploughs by R1 and R2 > 1 R3 Keyword: Asian & European moldboard ploughs, hyperboloid equation, constant ratios of hyperboloid equation.

The development of a coupled model (PCPF-SWMS) to simulate water flow and pollutant transport in Japanese paddy fields

A new coupled model (PCPF–SWMS) was developed for simulating fate and behavior of pollutant in paddy water and paddy soil. The model coupled the PCPF-1, a lumped model simulating pesticide concentrations in paddy water and 1 cm-surface sediment compartment, and the SWMS-2D, a finite element numerical model solving Richard's and advection-dispersion equations for solute transport in soil compartment. The coupling involved improvements on interactions of the water flow and the concentration the pollutant of at the soil interface between both compartments. The monitoring data collected from experimental plots in Tsukuba, Japan in 1998 and 1999 were used to parameterise and calibrate hydraulic functioning, hydrodynamic and hydrodispersive parameters of the paddy soil. The analysis on the hydraulic functioning of paddy soil revealed that the hard pan layer was the key factor controlling percolation rate and tracer transport. Matric potential and tracer monitoring highlighted the evolution of saturated hydraulic conductivity (KS) of hard pan layer during the crop season. KS slightly decreased after puddling by clay clogging and strongly increased after mid term drainage by drying cracks. The model was able to calculate residential time in every soil layers. Residential time of tracer in top saturated layers was evaluated to be less than 40 days. It took 60 days to reach the unsaturated layers below hardpan layer.

Influence of soil water on stress–strain behaviour of a compacting soil in semi-arid Kenya

Depending on the top and subsoil textures, semi-arid soils exhibit cohesive and frictional properties that are associated with the relatively high soil strength, bulk density and penetration resistance. The objective of this study was to gain the knowledge of mechanical properties of the compacting chromic luvisols in order to improve the design of tillage tools. Therefore, we applied critical state soil mechanics to study the stress–strain behaviour of the luvisols using triaxial tests under laboratory conditions. Field investigations involved random collection of undisturbed soil samples which were subjected to triaxial testing first by isotropic consolidation and compression and then triaxial shearing. Plots of deviatoric stress against axial strain were made to determine the soil shear strengths at the critical states over different soil water levels and the two soil depths of 0–20 cm for the plough and 20–40 cm for the hard pan layers, respectively. An exponential model used to fit the deviatoric stress–axial strain test data accurately predicted the trends. Soil water significantly influenced the shear strength, cohesion ( c′) and internal angle of friction ( ϕ′) and hence the mechanical behaviour of the luvisols. The regression equations developed showed that c′ and ϕ′ have quadratic relationships with soil water. The very high clay bonding strength in the subsoil (hard pan) layer resulted in high shear strength, bulk density and penetration resistance values for this soil layer. The increase in shear strength with decreasing water content affected the deviatoric stress–axial strain relationships between the upper and lower plastic limits of the sandy soil. Thus, as the soil dried, the soil ceased to behave in the plastic (ductile flow) manner and thus began to break apart and crumble. The crumbling was indicative of brittle failure. The transition stage from an increase to a decrease in c′ and ϕ′ values with soil water occurred in the soil water content range of 6–10%. Knowledge of stress–strain behaviour of compacting soils is of practical significance in the design of appropriate tillage tools for the specific soil type.

Tracking Sources of Unsaturated Zone and Groundwater Nitrate Contamination Using Nitrogen and Oxygen Stable Isotopes at the Hanford Site, Washington

The nitrogen and oxygen isotopic compositions of nitrate in pore water extracts from unsaturated zone (UZ) core samples and groundwater samples indicate at least four potential sources of nitrate in groundwaters at the U.S. DOE Hanford Site in south-central Washington. Natural sources of nitrate identified include microbially produced nitrate from the soil column (delta15N of 4 - 8 per thousand, delta18O of -9 to 2 per thousand) and nitrate in buried caliche layers (delta15N of 0-8 per thousand, delta 18O of -6to 42 per thousand). Isotopically distinctindustrial sources of nitrate include nitric acid in low-level disposal waters (delta15N approximately per thousand, delta 18O approximately 23%o) per thousandnd co-contaminant nitrate in high-level radioactive waste from plutonium processing (6'5delta1of 8-33 % o, per thousand18delta oO -9 to 7%0). per thousandThe isotopic compositions of nitrate from 97 groundwater wells with concentrations up to 1290 mg/L NO3- have been analyzed. Stable isotope analyses from this study site, which has natural and industrial nitrate sources, provide a tool to distinguish nitrate sources in an unconfined aquiferwhere concentrations alone do not. These data indicate that the most common sources of high nitrate concentrations in groundwater at Hanford are nitric acid and natural nitrate flushed out of the UZ during disposal of low-level wastewater. Nitrate associated with high-level radioactive UZ contamination does not appear to be a major source of groundwater nitrate at this time.

USING SITE-SPECIFIC SUBSOILING TO MINIMIZE DRAFT AND OPTIMIZE CORN YIELDS

Subsoiling is often required to alleviate soil compaction; however, deep tillage can be expensive andtime-consuming. If this tillage operation is conducted deeper than the compacted soil layer, energy is wasted. However, if thistillage operation is conducted shallower than the compacted soil layer, energy is again wasted, and plant roots may beprevented from penetrating the compacted layer. Technologies are now available that allow subsoiling to be conducted at thespecific depth of the compacted layer, which would conserve natural resources without sacrificing crop yields. An experimentwas conducted over four years in a field located in southern Alabama to evaluate whether the concept of site-specificsubsoiling (tilling just deep enough to eliminate the hardpan layer) would reduce tillage draft and energy requirements and/orreduce crop yields. Average corn (Zea mays L.) yields over this four-year period showed that site-specific subsoiling producedyields equivalent to those produced by the uniform deep subsoiling treatment while reducing draft forces, drawbar power, andfuel use.

Release, transport and attenuation of metals from an old tailings impoundment

The oxidation of sulfide minerals from mine wastes results in the release of oxidation products to groundwater and surface water. The abandoned high-sulfide Camp tailings impoundment at Sherridon, Manitoba, wherein the tailings have undergone oxidation for more than 70 a, was investigated by hydrogeological, geochemical, and mineralogical techniques. Mineralogical analysis indicates that the unoxidized tailings contain nearly equal proportions of pyrite and pyrrhotite, which make up to 60 wt% of the total tailings, and which are accompanied by minor amounts of chalcopyrite and sphalerite, and minute amounts of galena and arsenopyrite. Extensive oxidation in the upper 50 cm of the tailings has resulted in extremely high concentrations of dissolved SO 4 and metals and As in the tailings pore water (pH < 1, 129,000 mg L −1 Fe, 280,000 mg L −1 SO 4, 55,000 mg L −1 Zn, 7200 mg L −1 Al, 1600 mg L −1 Cu, 260 mg L −1 Mn, 110 mg L −1 Co, 97 mg L −1 Cd, 40 mg L −1 As, 15 mg L −1 Ni, 8 mg L −1 Pb, and 3 mg L −1 Cr). The acid released from sulfide oxidation has been extensive enough to deplete carbonate minerals to 6 m depth and to partly deplete Al-silicate minerals to a 1 m depth. Below 1 m, sulfide oxidation has resulted in the formation of a continuous hardpan layer that is >1 m thick. Geochemical modeling and mineralogical analysis indicate that the hardpan layer consists of secondary melanterite, rozenite, gypsum, jarosite, and goethite. The minerals indicated mainly control the dissolved concentrations of SO 4, Fe, Ca and K. The highest concentrations of dissolved metals are observed directly above and within the massive hardpan layer. Near the water table at a depth of 4 m, most metals and SO 4 sharply decline in concentration. Although dissolved concentrations of metals and SO 4 decrease below the water table, these concentrations remain elevated throughout the tailings, with up to 60,600 mg L −1 Fe and 91,600 mg L −1 SO 4 observed in the deeper groundwater. During precipitation events, surface seeps develop along the flanks of the impoundment and discharge pore water with a geochemical composition that is similar to the composition of water directly above the hardpan. These results suggest that shallow lateral flow of water from a transient perched water table is resulting in higher contaminant loadings than would be predicted if it were assumed that discharge is derived solely from the deeper primary water table. The abundance of residual sulfide minerals, the depletion of aluminosilicate minerals in the upper meter of the tailings and the presence of a significant mass of residual sulfide minerals in this zone after 70 a of oxidation suggest that sulfide oxidation will continue to release acid, metals, and SO 4 to the environment for decades to centuries.

Measurement and variation of site-specific hardpans for silty upland soils in the Southeastern United States

Soil compaction in the form of a hardpan layer restricts root growth and reduces crop yield throughout the Southeastern United States. However, the depth of this compacted soil layer varies significantly over landscapes. A sampling procedure was developed and evaluated to determine the depth of the hardpan layer in upland soils of Grenada silt loam soil type in this region. Cone index profiles taken with a multiple-probe soil cone penetrometer in three fields were used to measure the hardpan depth and to predict their spatial variation. Continuous treatments of these fields for several years included conventional tillage, no-tillage, segregated traffic, and random traffic. Conventional tillage systems were found to bring the hardpan significantly closer to the soil surface as compared to no-till systems; in trafficked row middles by 0.144 m, directly beneath the rows by 0.165 m, and in no-trafficked row middles by 0.094 m. Little difference in depth of hardpan was found between a no-till field subjected to random traffic and a field where traffic was segregated. However, the effect of this random traffic was found to dramatically reduce the predicted range (which is a geo-statistical measure of field variability) from 43.2 m for the no-till field in the trafficked row middle as compared to 12.4 m for the randomly trafficked no-till field. This may indicate that the depth of the hardpan layer in these soils is brought uniformly closer to the soil surface by the effect of traffic, especially when annual tillage is not practiced which could alter this hardpan depth. This information about the variation of the hardpan layer will greatly assist researchers who are interested in developing site-specific tillage technologies to remedy this soil compaction condition.